I agree in priciple to the “yashrus” argument, but I think there are two nafka minas between that argument and the “previous moral obligation” argument. Firstly, I think the level of anger shown is unjustified. Human nature is to find someone to blame for your issues, and when someone is inconvenienced, and certainly when injured, the tendency is to want to blame someone. The homeowner is the convenient and obvious target for essentially shirking what is not a true moral obligation, only an expected and proper chessed.
Also, I don’t think it’s reasonable to blame someone who is physically unable to shovel for not finding an expensive alternative. Do we really blame senior citizens and handicapped people for having their walks unshoveled? I would think not.